Immunoassays are widely used as methods for qualitatively or quantitatively measuring the presence of an analyte existing in a biological sample such as urine or blood. Of these immunoassays, an immunochromatography method is generally used with high frequency since its implementation is simple and enables short-time measurement.
The competitive reaction and the sandwich reaction are broadly used as immunoreactions to be employed in immunochromatography methods. In particular, the sandwich reaction is mainly employed for an immunochromatography method. In a typical example of the use of the sandwich reaction, the following procedures are performed to detect an analyte comprising an antigen in a sample. (1) A chromatographic medium having a reaction site is prepared by immobilizing a fine particle as a solid phase fine particle that has been sensitized with an antibody against an antigen that is an analyte on a chromatographic medium or by directly immobilizing the antibody on a chromatographic medium. (2) Meanwhile, a labeled antibody is prepared by sensitizing a labeled substance with an antibody capable of specifically binding to an analyte. (3) The labeled antibody is caused to migrate chromatographically on a chromatographic medium together with a sample.
The thus immobilized antibody is as an immobilized reagent at the detection site formed on the chromatographic medium by the above procedures. The labeled antibody specifically binds to the reagent via an antigen that is an analyte. As a result, the presence, absence, or the amount of an analyte in a sample is measured by visually determining the presence, absence, or the degree of signals generated when the labeled antibody is captured at the detection site.
When substances which act in ultratrace amounts (including bioactive substances or environmental pollutants such as toxins, hormones, or agricultural chemicals) are detected, the detection signal may sometimes be amplified. For example, when an enzyme such as alkaline phosphatase or peroxidase may be used as a labeling substance, an enzyme-linked coloring method can be used for signal amplification. When gold colloid is used as a labeling substance, silver amplification method can be used for signal amplification.
When signal amplification is generally carried out in immunochromatography method or the like, not only signal from labeling substance which was specifically bound to a solid phase but also signal from labeling substance which was non-specifically adsorbed are amplified. Therefore, suppression of non-specific adsorption is important object. For such purpose, after an antibody is immobilized, the solid phase is blocked with a non-specifically adsorption suppressing agent such as protein (for example, albumin, globulin, or casein) or polymer (for example, polyvinyl alcohol, or polyvinylpyrrolidone).
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